Method and apparatus for treatment of effluents from production plants of epoxy compounds

ABSTRACT

The invention relates to a process of abatement of the organic content of a depleted brine coming from epoxy compound production involving a vapour stripping step and a mineralisation with hypochlorite in two steps, at distinct pH and temperature conditions.

FIELD OF THE INVENTION

The invention relates to treatment and recycling of a processelectrolyte in the industrial synthesis of epoxy compounds and to therelevant production plant.

BACKGROUND OF THE INVENTION

Epoxy compounds such as propylene oxide and epichlorohydrin areessential components in the production of epoxy resins used for paintsand artefacts including sophisticated ones, such as carbonfibre-reinforced materials increasingly used in the aeronautic industry.

The manufacturing processes of epoxy compounds are based on the schemeillustrated hereafter, providing the reaction of an unsaturated organiccompound (indicated by the formula CH₂=CH—R wherein R generally denotesan alkyl or chloroalkyl group) with chlorine and alkali, for instancecaustic soda. The overall reaction takes place through a first step ofhypochlorous acid (HClO) generation, a second step of hypochlorous acidaddition to the double bond of the unsaturated compound with formationof the corresponding chlorohydrin (indicated as CH₂Cl—CH(OH)—R) and athird step of conversion of the chlorohydrin with caustic soda to formthe end product—indicated hereafter as CH₂—(O)—CH—R and expressed bystructural formula (I)—and sodium chloride (depleted brine).

Reference will be explicitly made in the following to plants providingthe coupling of the epoxy compound synthesis section to chlorine-causticsoda units, but it is understood that the same concepts apply to otherchlor-alkali units (for instance chlorine-caustic potash electrolysiscells).

The overall reaction scheme of the epoxy compound production plant isreported hereafter.

-   -   chlorine-caustic soda unit:

2NaCl+H₂O→Cl₂+2NaOH+H₂

-   -   epoxidation section:

Cl₂+H₂O→HClO+HCl

CH₂=CH—R+HClO→CH₂Cl—CH(OH)—R

CH₂Cl—CH(OH—R+NaOH→CH₂—(O)—CH—R+NaCl

HCl+NaOH→NaCl+H₂O

The reaction scheme indicates that chlorine and caustic soda are used ina 1:2 molar ratio.

The industrial processes of higher relevance relate to the production ofpropylene oxide, expressed by formula (I) with R=CH₃, and ofepichlorohydrin, expressed by formula (I) with R=CH₂Cl, wherein theunsaturated compounds employed are respectively propylene (CH₂═CH—CH₃)and allyl chloride (CH₂Cl—CH═CH₂).

As it will be shown in the following, epichlorohydrin may also bemanufactured by an alternative process based on the use of glycerol,CH₂(OH)—CH(OH)—CH₂(OH), whose availability at convenient price isrecently increasing. The process is articulated in three steps given bythe combination of chlorine and oxygen to form gaseous hydrochloric acid(HCl), the reaction of glycerol with hydrochloric acid with productionof dichlorohydrin (CH₂Cl—CH(OH)—CH₂Cl) and finally the conversion ofdichlorohydrin to epichlorohydrin and depleted brine by means of causticsoda:

-   -   chlorine-caustic soda unit:

2NaCl+H₂O→Cl₂+2NaOH+H₂

-   -   hydrochloric acid gas manufacturing unit:

H₂+Cl₂→2HCl

-   -   epoxidation section:

CH₂(OH)—CH(OH)—CH₂(OH)+2HCl→CH₂Cl—CH(OH)—CH₂Cl+2H₂ O

CH₂Cl—CH(OH)—CH₂Cl+NaOH→CH₂—CH—(O)—CH₂Cl+NaCl

The reaction scheme indicates that chlorine and caustic soda are used ina 1:1 molar ratio.

Chlorine, hydrogen and caustic soda are manufactured in a diaphragm- ormembrane-type chlorine-caustic soda unit installed upstream the epoxycompound production plant.

In all processes destined to produce epoxy compounds, particularly inthe case of propylene oxide and epichlorohydrin, it is important toaccomplish the recycling of the depleted brine to the upstreamchlor-alkali unit: in fact, if the outlet brine is sent, as is thepresent case, to an external treatment plant, the amount of lost sodiumchloride is about 100,000 t/y for a medium to big size capacity, with aconsequent heavy economic impact on the plant management. The recyclingof depleted brine is feasible however only provided the content ofresidual organic compounds (expressed in the following in terms ofchemical oxygen demand, COD) is previously abated. Such operation israther difficult to be carried out biologically due to the high salinecontent; moreover, being this treatment a typical low-intensive process,it would require huge volumes and surfaces, hardly compatible with thenormal demands of production sites.

Patent application US-20100219372-A1 provides the COD abatement ofdepleted brines for epichlorohydrin production to be carried out bycombining at least two treatments of different nature, among which ageneric electrochemical treatment, a chemical oxidation for instancewith chlorine and caustic soda and a crystallisation are listed. Theinventors observed that, from a practical standpoint, the lattertreatment is essential for obtaining an outlet brine which caneffectively be recycled to the indicated process, i.e. having a finalCOD not exceeding 40 mg/l of oxygen. The crystallisation step isnevertheless lengthy and laborious, entailing the separation of sodiumchloride crystals from the depleted brine with formation of a motherliquor, the redissolution of separated crystals to obtain clean brine, amore thorough crystallisation on a purge of the mother liquor and therecycling of the relevant salt. By combining such step with otherchemical and electrochemical treatments mentioned in the specification,the brine obtained has an acceptable quality in terms of organiccontent, but too rich in chlorates (with a typical concentration in theorder of magnitude of 1 g/l) and in chlorinated organic derivativeswhich are formed as a natural consequence of such treatments. Theconcentration of such by-products in the brine must be suitably adjustedby methods known in the art, for instance by adsorption on activecarbons (abatement of chlorinated by-products) and by injection ofsulphite in acidic environment (abatement of chlorates). The combinationof treatments suggested in US-20100219372-A1 provides excessively highvalues of such by-products, making the relevant abatement treatmentsextremely penalising.

It has thus been evidenced the need for processes of restoration ofdepleted brines in epoxy compound production plants characterised bysimplicity of operation, reduced size and reasonable cost.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

Under one aspect, the invention relates to a process of reduction of theorganic content of a depleted brine originated in the manufacturing ofepoxy compounds by oxidation of an organic raw material with theproducts of a chlor-alkali electrolysis unit, comprising a first removalof a substantial fraction of residual organic compounds by vapourstripping of the depleted brine optionally at pH adjusted between 3 and4 upon injection of a flow of water, followed by mineralisation (i.e.conversion down to carbon dioxide) by pre-oxidation with hypochlorite atpH 3.5 to 5 and at a temperature of 50-60° C. and final oxidation in thepresence of hypochlorite at pH 3 to 4 and at a temperature of 80 to 95°C. The term hypochlorite is used herein and in the following todesignate the hypochlorite species in salt form in equilibrium withhypochlorous acid at the relevant pH, as it will be evident to thoseskilled in the art. In particular, the vapour stripping step is used towithdraw the totality of volatile organic substances together with partof higher boiling ones; this can have the advantage of sensiblyrelieving the subsequent oxidation steps, reducing in particular theformation of chlorinated by-products during such phases. In oneembodiment, the vapour stripping can abate the COD of a typical spentbrine (normally higher than 10,000 and sometimes exceeding 30,000 mg/lof oxygen) down to a value of 2,000-4,000 mg/l of oxygen. Such residualquantity is suitable for being subjected to an oxidation treatment withhypochlorite; the inventors observed that carrying out such oxidation intwo steps—that is a pre-oxidation step at slightly higher pH and lowtemperature, followed by thorough oxidation at lower pH and highertemperature—has the advantage of minimising the formation of chlorates(with a typical concentration of the order of magnitude of 0.1 g/l orlower) and of chlorinated by-products. The pre-oxidation is in factcapable of further reducing the COD, which in one embodiment is 800 to1,500 mg/l of oxygen at the outlet of the pre-oxidation step. In oneembodiment, the pre-oxidation step is effected by feeding chlorine andalkali, for instance caustic soda, optionally produced in the samechlor-alkali electrolysis unit providing the reactants for the oxidationof the organic raw material. This has the advantage of manufacturing thehypochlorite required for the pre-oxidation by means of reagents alreadypresent on site. In one embodiment, the pre-oxidation step is effectedin an alkali brine electrolysis cell of undivided type commonly used inthe manufacturing of hypochlorite.

The final oxidation step is carried out in the presence of hypochloriteat pH 3 to 4 and at a temperature of 80 to 95° C.; such process stepprovides a fresh brine, which in one embodiment is characterised by aCOD not higher than 40 mg/l of oxygen, with a chlorate concentration nothigher than 0.1 g/l and a moderate content of chlorinated by-products.The concentration of the two by-products may optionally be adjustedbefore recycling the brine to the plant by absorption treatments onactive carbons and by injection of sulphite at pH controlled in anacidic range; such operations are made fully feasible by the muchdecreased amounts involved with respect to the processes of the priorart. Similarly to the case of pre-oxidation, in one embodiment the finaloxidation step is effected by feeding chlorine and alkali, for instancecaustic soda, optionally produced in the same chlor-alkali electrolysisunit providing the reactants for the oxidation of the organic rawmaterial. In one embodiment, the final oxidation step is effecteddirectly in the chlor-alkali electrolysis unit fed with fresh brine,provided the latter consists of a diaphragm cell. The term diaphragmcell is used herein to mean an electrolysis cell equipped with anon-asbestos type diaphragm separator comprising fluorinated polymerfibres and optionally inorganic materials such as zirconium oxide, aswould be known to those skilled in the art. The inventors in factsurprisingly observed that such brine stream can be fed into the anolyteof diaphragm cells without any problem, since the semi-permeable natureof the diaphragm causes hypochlorite to be locally produced within theanolyte itself, such hypochlorite comprising a remarkable fraction ofhypochlorous acid in view of the acidic environment (pH 3-4), in anamount such that it oxidises a major part of residual organics in situ.This solution is conversely not applicable if the chlor-alkalielectrolysis unit is of different type, for instance in the case ofcells equipped with ion-exchange membranes as the separator (membranecells): in fact, feeding a brine with a COD of few hundreds mg/l ofoxygen as obtainable in the pre-oxidation step according to theinvention would bring about serious malfunctioning of ion-exchangemembranes and of anodes in time. Thus, the final oxidation step in thiscase must be carried out in a separate unit, upstream the cell, byfeeding chlorine and alkali as already mentioned or, in a furtherembodiment, in a unit consisting of an alkali brine electrolysis cell ofundivided type commonly used in the manufacturing of hypochlorite.

Under another aspect, the invention relates to a synthesis plant of anepoxy compound comprising a chlor-alkali electrolysis unit fed withfresh brine, a depleted brine vapour stripping unit, a unit ofpre-oxidation with hypochlorite. In one embodiment, the pre-oxidationunit consists of an alkali brine electrolysis cell of undivided typecommonly used in the manufacturing of hypochlorite. In one embodiment,the chlor-alkali electrolysis unit consists of an electrolysis cellequipped with a non-asbestos type diaphragm separator comprisingfluorinated polymer fibres. In one alternative embodiment, the plantfurther comprises a final oxidation unit consisting of a reactor fedwith chlorine and caustic soda and the chlor-alkali electrolysis unitconsists of a membrane-type electrolysis cell.

Some implementations exemplifying the invention will now be describedwith reference to the attached drawing, which has the sole purpose ofillustrating the reciprocal arrangement of the different elementsrelatively to said particular implementations of the invention; inparticular, drawings are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scheme of production of propylene oxide according to theprior art comprising a chlor-alkali electrolysis unit in which depletedbrine is sent to an external treatment plant.

FIG. 2 shows a scheme of production of epichlorohydrin according to theprior art comprising a chlor-alkali electrolysis unit in which depletedbrine is sent to an external treatment plant.

FIG. 3 shows a scheme of production of propylene oxide according to theinvention comprising a membrane chlor-alkali electrolysis unit.

FIG. 4 shows a scheme of production of epichlorohydrin from glycerolaccording to the invention comprising a diaphragm chlor-alkalielectrolysis unit.

DETAILED DESCRIPTION OF THE DRAWINGS

The scheme shown in FIG. 1 indicates that a propylene oxide productionplant according to the prior art comprises a chlor-alkali unit 7, forinstance a diaphragm- or membrane-type chlorine-caustic soda cell, fedwith brine obtained by dissolving a solid salt 6, for instance sodiumchloride, into water 5, with optional reintegration of recycled salt 14.The products of the chloralkali unit consist of chlorine 1, catholyte2—which in the case of a diaphragm chlorine-caustic soda cellindicatively contains 15% NaOH and 15% NaCl and in the case of amembrane chlorine-caustic soda cell contains a 32% by weight aqueoussolution of caustic soda—and hydrogen 4. Chlorine 1 and catholyte 2,optionally diluted with water 5, are fed to the propylene oxide unit 10,where they react with propylene 9 according with the above describedreaction scheme. The reaction mixture is sent to a separation unit 13which extracts propylene oxide 11 and discharges depleted brine 12corresponding in this case to the whole amounts of chlorine and causticsoda produced in the chlor-alkali unit. In this scheme it is assumedthat depleted brine 12, containing sensible amounts of organicsubstances besides 20-25% NaCl, is sent to an external treatment forcompliance with the environmental norms applicable to industrial wastewaters. In the case of a diaphragm electrolysis unit, catholyte 2,instead of being fed directly to reactor 10, may be sent to evaporator17 from which solid salt to be recycled 14, concentrated NaOH 15 to beinjected into reactor 10 after dilution with water 5 and condensate 16are extracted. This alternative allows avoiding introducing NaCltogether with NaOH into reactor 10 and is used in case the electrolysisunit is oversized with respect to the requirement of propylene oxideproduction: in this case additional concentrated caustic soda 15 andchlorine 1 are sent to other final users.

The scheme shown in FIG. 2 refers to an epichlorohydrin production plantutilising glycerol as raw material. The plant comprises a diaphragm- ormembrane-type chlor-alkali electrolysis unit 7 fed with imported solidsalt 6 and recycled solid salt 14 dissolved in water 5. The products ofthe chlor-alkali electrolysis unit are the same as the case shown inFIG. 1. In the case of a diaphragm cell unit, the fraction of catholyte2 exceeding the requirement of epichlorohydrin production is fed to anevaporation-crystallisation section 17 from which are extracted solidsalt to be recycled 14, concentrated NaOH 15 to be exported andcondensate 16. There is also the possibility of feeding all of catholyte2 to the evaporation-crystallisation unit 17: in such case, the requiredfraction of concentrated NaOH 15 is sent to saponifier 23 after dilutionwith water 5 while the fraction exceeding the requirement ofsaponification is exported. Such an alternative avoids feeding sodiumchloride together with caustic soda into saponifier 23.

In the case of a membrane cell unit, the fraction of catholyte 2exceeding the requirement of epichlorohydrin production is fed to aconcentration section (not shown in the figure) from which NaOH isextracted at a commercial weight concentration of 50%.

The evaporation-crystallisation and concentration units are also neededin case the electrolysis unit is oversized with respect to therequirement of epichlorohydrin production: in this case additionalconcentrated caustic soda and chlorine are sent to other final users.

Chlorine and hydrogen are combined in combustion unit 18 where anhydrousHCl 27 sent to subsequent unit 20 is produced: here dichlorohydrin 28 isobtained by reacting gaseous hydrochloric acid with glycerol 19.Dichlorohydrin is reacted with catholyte in saponifier 23 from whichepichlorohydrin 21 and depleted brine 12, containing relevant amounts oforganic substances besides 20-25% of NaCl, are extracted. Depleted brine12 is sent to an external treatment.

The scheme shown in FIG. 3 illustrates an embodiment of the presentinvention applicable to propylene oxide plants comprising amembrane-type chlor-alkali electrolysis unit 7, in the followingreferred to as chlorine-caustic soda unit. In this case depleted brine12, separated from propylene oxide 11, has a typical COD of 2,500-3,000mg/l of oxygen and must be treated to a target value of 20-40 mg/l ofoxygen in order to be recycled while preventing membrane decay andpossible anode malfunctioning. For this purpose, depleted brine 12 isfed to a vapour stripping unit 29. The operation is carried out so as toconcentrate the depleted brine to near saturation, preferably withoutreaching the stage of solid salt separation. Inventors observed thatvapour stripping, particularly if carried out adjusting pH around 3-4 byhydrochloric acid addition, allows strongly decreasing COD: by operatingin this range with an outlet brine having a COD of about 2,500-3,000mg/l of oxygen a solution with a residue of about 1,000-1,500 mg/l ofoxygen can be obtained. It was found that the residual COD depends,besides the initial COD, on the amount of water 5 injected intocatholyte 2: such amount of water dictates in fact the vapour flow-ratein 29 and thus the efficiency of the stripping action. Additional watermay optionally be injected directly into stripping unit 29. The solutionat the vapour stripping outlet 29 is subsequently fed to a pre-oxidationunit 24 supplied in this case with chlorine and caustic soda at 1:2molar ratio with a 2-4 stoichiometric excess with respect to organics tobe abated: pre-oxidation unit 24 operates at pH 3.5 to 5 and at atemperature of 50 to 60° C. In these conditions it was possible toeasily decrease the residual COD down to values of 400-600 mg/l ofoxygen, with an extremely reduced content of chlorates and chlorinatedby-products. The solution exiting pre-oxidation unit 24 is then fed tofinal oxidation unit 25 consisting in this case of an undivided typeelectrolyser for hypochlorite solution manufacturing, working in optimumoperating conditions at pH adjusted in the range 3-4 and at atemperature of 80-95° C. In these conditions, an outlet brine with a CODvarying between 20 and 40 mg/l of oxygen could be obtained from thefinal oxidation unit 25, compatible with the correct operation ofmembranes and anodes of the membrane electrolysis unit.

FIG. 4 shows an embodiment of the invention relative to anepichlorohydrin manufacturing plant comprising a-type chlor-alkalielectrolysis unit 7, in the following referred to as chlorine-causticsoda unit, of the type equipped with a non-asbestos diaphragm based onfluorinated polymer fibres. In this case depleted brine 12, typicallycharacterised by high COD values, for instance 10,000-30,000 mg/l ofoxygen, is sent as first treatment step to stripping unit 29. Inventorscould detect residual COD values in outlet solution 14 below 4,000 mg/lof oxygen and always comprised between 2,000 and 3,000 mg/l of oxygen bymaintaining the pH in the range 3-4 during the stripping step and byinjecting additional water directly into stripping unit 29. The outletsolution of vapour stripping unit 29 is subsequently sent to apre-oxidation unit 24 fed with chlorine and caustic soda at 1:1 molarratio with a 2-4 stoichiometric excess with respect to organics to beabated: pre-oxidation unit 24 operates at pH 3.5 to 5 and at atemperature of 50 to 60° C. In these conditions it was possible toeasily decrease the residual COD down to values of 800-1,000 mg/l ofoxygen, with an extremely reduced content of chlorates and chlorinatedby-products. The solution exiting pre-oxidation unit 24, added with therequired salt 6 and water 5, is then fed to a final oxidation unitcoinciding in this case with diaphragm-type chlorine-caustic soda unit7: by maintaining the pH of the diaphragm cell unit anodic compartmentsat 3-4 and the temperature at 90-95° C., is was then possible to obtaincaustic soda 2 at the outlet with a residual COD of only 20-40 mg/l ofoxygen, with no significant build-up of chlorinated by-products andchlorates in the production cycle. Inventors further observed that uponby-passing pre-oxidation unit 24 from the cycle and carrying out asingle stage oxidation inside diaphragm-type chlorine-caustic soda unit7, the COD of caustic soda at the outlet is never lower than 500-1,000mg/l of oxygen, moreover with a progressive build-up of chlorates andchlorinated by-products.

The previous description shall not be intended as limiting theinvention, which may be used according to different embodiments withoutdeparting from the scopes thereof, and whose extent is solely defined bythe appended claims.

Throughout the description and claims of the present application, theterm “comprise” and variations thereof such as “comprising” and“comprises” are not intended to exclude the presence of other elements,components or additional process steps.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention before the priority date of each claim of thisapplication.

1. A process of reduction of the organic content of a waste streamconsisting of a depleted brine in a synthesis plant comprising thefollowing sequential steps: a) injecting a flow of water into thedepleted brine to be treated and vapour stripping; b) pre-oxidating withhypochlorite at pH 3.5 to 5 and at a temperature of 50 to 60° C.; and c)oxidating in the presence of hypochlorite at pH 3 to 4 and at atemperature of 80 to 95° C. to obtain obtaining a fresh brine, whereinthe depleted brine is obtained from a plant of synthesis of an epoxycompound by oxidation of an organic raw material by means of theproducts of a chlor-alkali electrolysis unit fed with fresh brine. 2.The process according to claim 1 wherein said depleted brine has a CODhigher than 10,000 mg/l of oxygen at the inlet of step a) and of 2,000to 4,000 mg/l of oxygen at the outlet of step a).
 3. The processaccording to claim 1 wherein said vapour stripping step takes place atpH adjusted between 3 and
 4. 4. The process according to claim 1 whereinsaid depleted brine has a COD of 400 to 1,500 mg/l of oxygen at theoutlet of step b).
 5. The process according to claim 1 wherein saidfresh brine at the outlet of step c) has a COD not higher than 40 mg/lof oxygen.
 6. The process according to claim 1 wherein said organic rawmaterial is selected from the group consisting of propylene, allylchloride and glycerine and said epoxy compound is propylene oxide orepichlorohydrin.
 7. The process according to claim 1 wherein saidpre-oxidation step with hypochlorite is carried out by feeding chlorineand alkali.
 8. The process according to claim 1 wherein saidpre-oxidation step with hypochlorite is carried out within anelectrolysis cell of the undivided type.
 9. The process according toclaim 1 wherein said final oxidation step is carried out in anelectrolysis cell.
 10. The process according to claim 9 wherein saidelectrolysis cell is an alkali brine electrolysis cell of the undividedtype.
 11. The process according to claim 9 wherein said final oxidationstep is carried out directly within the chlor-alkali electrolysis unitfed with fresh brine, consisting of an electrolysis cell equipped with anon-asbestos diaphragm separator comprising fluorinated polymer fibers.12. The process according to claim 1 followed by one or more of thefollowing simultaneous or sequential steps: d) adjusting theconcentration of chlorinated by-products by absorption on active carbon;e) adjusting the concentration of chlorate by injection of sulphites atpH regulated in an acidic range, wherein the fresh brine containschlorate and chlorinated by products.
 13. A synthesis plant of an epoxycompound comprising means for reducing organic content of depleted brineof said synthesis plant, said reducing means comprising a chlor-alkalielectrolysis unit fed with fresh brine, a depleted brine vapourstripping unit, a unit of pre-oxidation with hypochlorite adapted tocarry out said pre-oxidation at pH 3.5 to 5 and at a temperature of 50to 60° C.
 14. The plant according to claim 13 wherein said chlor-alkalielectrolysis unit fed with fresh brine consists of an electrolysis cellequipped with a non-asbestos diaphragm separator comprising fluorinatedpolymer fibers.
 15. The plant according to claim 13 further comprising afinal oxidation unit consisting of a reactor fed with chlorine andcaustic soda, wherein said chlor-alkali electrolysis unit fed with freshbrine consists of a membrane-type electrolysis cell.
 16. The plantaccording to claim 13 wherein said pre-oxidation unit consists of anelectrolysis cell of the undivided type.